The present invention relates to a discharge-lamp lighting circuit for lighting a plurality of discharge lamps and more particularly to the art of simplifying a starting circuit configuration as well as guaranteeing the performance of turning on each discharge lamp.
In case where a discharge lamp (metallic halide lamp) is applied to, for example, the light source of a vehicle lamp, there is a known lighting circuit configuration comprising a DC power supply, a power supply portion (switching power supply circuit), a DC-AC converter circuit, a starting circuit and so on.
A starting circuit (so-called starter circuit) for generating a high-tension starting pulse is needed when a discharge lamp is started and in case where a plurality of discharge lamps are turned on by a lighting circuit common to these discharge lamps, each starting circuit is provided corresponding to each discharge lamp.
A typical starting circuit configuration comprises a capacitor for accumulating electric charge and a switch element (a thyristor element or a spark gap element) for discharging the charge, both of the capacitor and the switch element being provided in a primary-side circuit including the primary winding of a transformer (trigger transformer), and the pulse generated by the conduction of the switch element is boosted by the transformer, so that the starting pulse is supplied from the secondary winding to a discharge lamp.
In the form of providing the starting circuit to each discharge lamp, the number of parts for use increases in proportion to an increase in the number of discharge lamps and this is problematical in view of size and cost reduction.
In case where such a switch element is provided in the primary-side circuit of the transformer forming the starting circuit, for example, it is necessary to provide switch elements equal in number to discharge lamps.
In order to provide a starting circuit for common use, the primary-side circuit of transformers is set for common use; for example, the primary-side circuit of the transformers is formed with one circuit including a primary winding, a capacitor and a switch element, and simultaneously secondary windings equal in number to discharge lamps are provided on the secondary side of the transformers. Then the number of parts for use can be curtailed by supplying the staring pulse to the individual discharge lamp via each secondary winding.
According to this method, however, it has been proved necessary to take the following matters into consideration so that the discharge lamp is turned on safely and certainly.
(1) The capacitance of the capacitor forming the primary-side circuit of the transformers is capacitance resulting from multiplying the capacitance which is necessary when one discharge lamp is turned on by the number of discharge lamps and the characteristics immediately after the breakdown of the discharge lamps conform to one another.
(2) The capacitance of the capacitor forming the primary-side circuit of the transformers is equal to or greater than twice as capacitance resulting from multiplying the capacitance which is necessary when one discharge lamp is turned on by the number of discharge lamps.
When the subject (1) is adopted first, there remains a problem arising from checking a reduction in costs to the extent that the capacitance of the capacitor increases as far as the first half portion of the reason is concerned. With regard to the second half portion of the reason (that the characteristics immediately after the breakdown of the discharge lamps need to conform to one another), it is owning to the fact that the energy stored in the capacitor has to be equally distributed. More specifically, the energy is supplied to the discharge lamp in an instant the breakdown of the discharge lamp occurs and plays an important role in controlling stably shifting toward turning on the discharge lamp. Unless the characteristics of the discharge lamps conform to one another in case where the breakdown of the whole discharge lamp occurs when the starting pulse is supplied, there develops partiality in that greater energy is distributed to a certain discharge lamp, whereas less energy is distributed to another. Consequently, the discharge lamp that has received less energy can hardly stably shift to its ON state. In order to obviate such a condition of the discharge lamp, it is only needed to selectively use discharge lamps whose characteristics conform to one another. However, difficulties in sorting man-days and schedule control still remain to be solved.
The subject (2) is intended to supply energy sufficient to make even the discharge lamp that has received less energy stably shift to its ON state on the assumption that the distribution of energy to each discharge lamp is unequal. In other words, the capacitor is stored with a somewhat larger quantity of energy beforehand and caused to discharge the energy by the conduction of the switch element. Notwithstanding, this method requires a large-capacity capacitor to be fully recharged in a short time so that a high-tension pulse is generated. Moreover, it is needed to not solely add a regulating or control circuit for stably turning on each discharge lamp to the discharge lamp supplied with greater energy but also employ a large-capacity capacitor. The problem in this case is that the lighting circuit tends to become large-sized, which results in an increase in costs.
An object of invention is to not only improve lighting performance of a discharge-lamp lighting circuit by allowing a plurality of discharge lamps to stably shift to their ON state after the discharge lamps are started but also decrease the size and cost of the discharge-lamp lighting circuit.
In order to solve the problem above according to the invention, a discharge-lamp lighting circuit for turning on a plurality of discharge lamps, comprising a starting circuit for supplying starting pulses to discharge lamps has the following configuration.
The starting circuit is provided with transformers having primary and secondary windings equal in number to the discharge lamps, the discharge lamps being connected to the respective secondary winding.
One capacitor is connected to the primary winding of each transformer and a circuit including the capacitors and one switch element is provided on the primary side of the transformers.
When the accumulated charge in each capacitor is discharged via the primary winding of the transformer as the switch element conducts, the starting pulse is generated and supplied to each discharge lamp via the secondary winding of the transformer.
According to the invention, the primary winding of each transformer added to each discharge lamp has a capacitor so as to secure accumulated energy necessary for the discharge lamp to stably shift to its ON state and by providing the switch element for common use in the primary-side circuit of the transformers, the size and cost of the discharge-lamp lighting circuit can be decreased.